Heretofore, various communication systems have been discussed and developed. A spread-spectrum communication system is known as one of these systems.
The spread-spectrum communication system is such that a transmitter sends a narrow-band signal indicative of data or voice by spreading out the bandwidth, using a pseudonoise code (PN code), and that a receiver reproduces the original signal by inversely spreading out the wideband received signal into the original narrow-band signal. The spread-spectrum communication system is recently remarked as a very reliable communication system because of such advantages as stability against external interference or noise and high privacy.
The greatest point of the spread-spectrum communication system is a correlator at a receiver side. In a wireless spread-spectrum communication, a correlator presently recognized to be most simple and reliably is an apparatus using a surface acoustic wave (SAW).
As SAW correlators, there are tapped delay line types and convolver types in general. Such a tapped delay line type, although simple in arrangement and generally excellent in efficiency, is largely affected by the temperature factor of a substrate. In contrast, such a convolver type, although not affected so much by changes in the temperature, is generally low in efficiency. However, while the tapped delay line type is fixed in code, the convolver type can change its code as desired.
Therefore, as far as the efficiency is practically acceptable, the convolver type correlator is much easier to use.
FIGS. 12(A) and 12(B) show an arrangement of a spread-spectrum communication apparatus using a prior art SAW convolver. In a transmitter side of FIG. 12(A), a mixer 1, for example, effects binary-phase modulation of a carrier from an oscillator 2, using a signal (digital in this case) to be sent, to first effect primary narrow-band modulation. Subsequently, a mixer 3, for example, effects binary-phase modulation using a PN code of a PN code generator 4 having a fast bit rate of a much wider band than an information signal to be sent, so as to spread out the spectrum and transmit it from the antenna 7, after passing it through a band-pass filter 5 and an amplifier 6.
In a receiver side of FIG. 12(B), a spread-spectrum signal received by an antenna 7' is first high-frequency-amplified by an amplifier 9 and band-pass filters 8 and 10, and is subsequently applied to a SAW convolver 13.
The SAW convolver 13 is supplied with a reference signal which is made by a mixer 12 by binary-phase-modulating a carrier from an oscillator 11 which oscillates the same carrier frequency as a received signal input of the SAW convolver, using a PN code generator for generating a PN code which is time-inversed with respect to the PN code 4 of the transmitter side.
The PN code used as the reference signal is held in synchronization with the received PN code from the transmitter side by an envelope detector 16, a synchronizing circuit 17 and a PN code control circuit 18. At this time, an output from the convolver 13 (whose frequency is 2fc, twice the input carrier), while maintained in synchronization with the carrier of the received signal by a synchronizing circuit (its center frequency is 2fc), first passes through a mixer 21 via a band-pass filter 14 and an amplifier 15 and is subsequently sent to the envelope detector 16 for the purpose of the aforementioned PN code synchronization. When the PN code and the carrier synchronize with each other, primary data of baseband information is obtained from a data demodulating circuit 22.
In the prior art spread-spectrum communication apparatus, however, synchronization of the PN code and the carrier is indispensable, and this makes it difficult to simplify the receiver arrangement. This is a serious problem for practical use thereof.